1. Field of the Invention
The present invention relates to a color filter, a liquid crystal display device including the color filter, a transflective liquid crystal display device which has combined characteristics of transmissive liquid crystal display and reflective liquid crystal display, and a method for making a color filter.
2. Description of the Related Art
At present, liquid crystal display devices are used in various fields, such as for notebook PCs, mobile information terminals, desktop monitors, and digital cameras, because of their characteristics, such as their low weight, thinness, and low power consumption. With respect to liquid crystal display devices using backlights, in order to reduce the power consumption, there have been demands for higher utilization of light from the backlights and also color filters with higher transmittance. Although the transmittance of the color filters is improving year after year, it is not possible to expect a large decrease in power consumption due to an improvement in the transmittance.
Recently, a reflective liquid crystal display device which does not require a backlight light source having high power consumption has been developed, and it has been disclosed that a large reduction in power consumption is obtained, i.e., approximately one-seventh that of a transmissive liquid crystal display device (Nikkei Microdevices Flat Panel Display 1998 Yearbook, p. 126).
Although a reflective liquid crystal display device has the advantages of lower power consumption compared to a transmissive liquid crystal display device and of excellent outdoor visibility, the reflective liquid crystal display device has problems in that the display becomes dark and the visibility strongly deteriorates in places where a sufficient intensity of ambient light is not available. In order to make display visible even in a dark environment, two devices provided with light sources have been disclosed. One is a liquid crystal display device in which a backlight is provided as a light source and the reflection layer in each pixel has a cutout so that the device partially works in the transmissive display mode and partially works in the reflective display mode, i.e., a so-called transflective liquid crystal display device (disclosed, for example, in Fine Process Technology Japan '99, Professional Technology Seminar Paper A5), and the other is a reflective liquid crystal display device provided with a front light.
In a transflective liquid crystal display device provided with a backlight, since a transmissive area using light from the backlight and a reflective area using ambient light coexist in one pixel, it is possible to perform display with high visibility regardless of the ambient light intensity. However, when a color filter having a conventional structure shown in FIG. 3, in which a reflective area and a transmissive area are not specifically defined and color characteristics in one pixel are uniform, is used, it is difficult to obtain vivid transmissive display. Specifically, when the vividness (color purity) of transmitted light is improved, the color purity of reflected light is also increased, and brightness which trades off with the color purity is extremely decreased, resulting in insufficient visibility. The problem is caused by the fact that in transmissive display, light from the backlight passes through the color filter once, while, in reflective display, ambient light passes through the color filter twice, i.e., when ambient light enters and when ambient light is reflected. Additionally, in a transflective liquid crystal display device, since the light source in the transmissive display mode is a backlight and the light source in the reflective display mode is ambient light, besides the color purity, the color tone also changes. The reason for this is that ambient light, as represented by a D65 light source, has a continuous spectrum, while light from the backlight has peaks of the spectrum at specific.
In order to solve the problems described above, a so-called “thickness-adjustment method” is disclosed in Japanese Unexamined Patent Publication No. 2001-33778 in which a transparent resin layer is formed in the reflective area so that transmissive display and reflective display have the same density of color (color purity), and thus brightness in reflective display is improved. FIG. 4 is a sectional view which schematically shows a color filter for a transflective liquid crystal display device fabricated by a known thickness-adjustment method. A transparent resin layer 3 is formed in a reflective area 6, and the thickness of a color layer 5 in the reflective area 6 is smaller than the thickness of the color layer 5 in a transmissive area 7. In this method, with respect to the color purity and brightness, the difference between transmissive display and reflective display can be decreased. However, it is not possible to correct the difference in color tone due to the fact that the light source in transmissive display is the backlight and the light source in reflective display is ambient light.
As a method for fabricating the transmissive area and the reflective area so as to have desired characteristics with respect to display (color purity, brightness, and color tone), use of a color filter shown in FIG. 5 may be mentioned, in which appropriate color layers are used for the individual transmissive areas and/or the reflective areas. In such a method (six-color application method), since the color purity and brightness can be changed in any way in each area, desired transmissive display and reflective display can be achieved. However, in photolithography, which is currently predominantly used for color filter manufacturing, at least two coloring agents must be applied to form pixels of one color, and in order to form pixels of three colors, red, green, and blue, photolithography must be performed at least two times for the individual colors, i.e., at least six times in total, resulting in an increase in the number of fabrication steps.
That is, in the conventional methods, it is not possible to obtain desired display characteristics (color purity, brightness, and color tone) for transmissive display and reflective display while fabricating a color filter inexpensively without increasing the number of fabrication steps.